1. Field of the Invention
The present invention related to a surface treatment method for a metal material, and a mold that is treated by the surface treatment method. More specifically, the present invention relates to a method for the formation of a carbon film on a surface of a metal material and a mold that has a cavity surface with a carbon film formed thereon.
2. Description of Related Art
A specific treatment (surface treatment) is sometimes performed on a surface of a mold (in particular, casting mold or forging mold) for such purposes as preventing seizure. As one method, a surface treatment method in which a nitrided layer is formed in a superficial layer of a metal material (mold) and a carbon film is formed on the nitrided layer has been developed (for example, Japanese Patent Application Publication No. 2010-36194 (JP 2010-36194 A) and Japanese Patent Application Publication No. 2008-105082 (JP 2008-105082 A). In the method that is disclosed in JP 2010-36194 A and JP 2008-105082 A, a nanocarbon film formation process in which the mold is subjected to a heat treatment in the presence of acetylene gas and hydrogen sulfide gas in a nitriding atmosphere is carried out. The term “nanocarbon film” refers to a carbon film which contains at least one nanocarbon material that is selected from the group that consists of carbon nanocoils, carbon nanotubes and carbon nanofilaments. A nitrided layer is formed in the vicinity of the surface (in a superficial layer) of the mold and a nanocarbon film is formed on the nitrided layer by a nanocarbon film formation process. Here, a compound layer is formed between the nitrided layer and the nanocarbon film. The term “nitrided layer” refers to a layer that is formed when nitrogen penetrates inside a superficial layer of a mold that is made of an iron or an alloy which contains iron.
It is believed that, in the surface treatment method that is disclosed in JP 2010-36194 A and JP 2008-105082 A, the hydrogen sulfide gas reacts with iron (Fe) that is contained in the metal material to form an Fe—S compound and a nanocarbon film is formed on a surface of the metal material by the catalytic action of the Fe—S compound. When hydrogen sulfide gas is contained in the reaction gas, a nanocarbon film is formed on a surface of the metal material. However, a reaction system that uses hydrogen sulfide gas requires high equipment and production costs.